Scientific Forecaster Discussion

Synopsis...a warm front will continue to lift northward over the area this morning with snow turning to rain along the front. Rainy weather is expected to continue for much of the weekend as an upper level low will pass over western Washington Saturday and into Sunday. Temperatures will remain mild this weekend before becoming colder for the first half of next week.

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Short term...transition from snow over to rain is occurring rather slowly this morning...but is occurring as obs from Chehalis and Centralia show rain. Current radar shows that precip activity is becoming a bit more scattered...particularly over southern third of the County Warning Area. Main forecast issue this morning is almost more of a nowcasting issue...anticipating the speed of the transition over to rain for portions of County Warning Area that are still seeing snow. Hrrr seems like a good tool for this...putting the transition to rain in olm between 3 and 4 am...in Seattle between 4 and 5 am and fairly widespread by 7 am this morning. All that being said...inherited timing looks to be on track and will opt not to alter until reality says otherwise.

Once precip becomes rain it looks to stick around for the weekend. This particular round of rainfall will linger for much of the day today and into tonight before becoming more scattered by Saturday morning. This will result in continued snow accumulations in the mountains. Given the expected pacing of accumulations...inherited Winter Weather Advisory for snow looks good...as 12 hour accumulations fall right in step with 12 hour advisory thresholds.

Upper level low pressure looks to move toward western Washington Saturday afternoon...bringing another round of organized precip to the area. The low itself appears to be fairly progressive...but lingering shortwave disturbances will keep rain in the forecast through Sunday afternoon before things start to scatter out. Smr

Long term...a frontal system looks to move through western Washington Monday with a return to colder temperatures behind it. Although the origins of the system are over the Pacific...making it seem mild...northeasterly surface pressure gradients look to open The Gates for more Arctic air thanks to Fraser outflow. Current model solutions hint at the Prospect of tightest surface pressure gradients occurring as the upper level system moves through...raising some questions as to what for precip might take during this period. With Arctic air entering...as opposed to having already been in place...leaning more toward rain at this time. Granted...it may be a moot point...as this alignment seems to have a pretty small window since chances of precip diminish in the evening and gone by Monday night. Cold and dry conditions are expected for the first half of next week as subfreezing low temperatures look to return. Smr

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Aviation...weak high pres aloft will move slowly east, allowing an upper level low to approach the British Columbia coast. An occluded front over far southwest Washington will continue moving slowly north-northeast, and is anticipated to reach Whatcom County late tonight. Wly flow aloft will strengthen during the day. The low level flow will grdlly become southerly across the entire area by late tonight.

Ksea...it looks like periods of light snow will will continue before transitioning to rain about 1500 UTC (7 am pst). There is a possibility of brief -fzra or sleet before the switch to just plain rain. Additional snowfall amount is anticipated to be less than an inch. Meanwhile, expect generally MVFR conditions today. Winds will be east or southeast near 10 knots or less.

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Marine... an occluded front extended from a 992 mb low off the northern tip of Vancouver Island southeast to the southwest Washington coast. This front will continue moving slowly north, reaching the U.S. Georgia Strait late tonight. The flow will become southerly behind the front.

A trough or weak cold front will move across the area on Saturday for increased onshore or westerly flow. Onshore flow will persist on Sunday before becoming northerly on Monday.